Jökull


Jökull - 01.12.1987, Side 7

Jökull - 01.12.1987, Side 7
trations in the waters below Landmannalaugar are thus attributed to dissolution from the acid volcanics over- lying the presumed basalt sheet intrusion heat source. As for other geothermal reservoir waters in Iceland associated with acid volcanics, fluoride concentrations are relatively high in the sodium-chloride type waters (5-25 ppm) and seem to be controlled by fluorite solu- bility in the reservoir (Arnórsson et al. 1983). Compositional features of the sodium-chloride waters emerging in boiling hot springs are, apart from chloride and fluoride, similar to those of waters discharged from drillholes in basaltic terrain in different parts of Iceland. Arnórsson etal. (1983) have shown that the composition of the drillhole waters is governed by chemical equilib- num between solutes and alteration minerals. The mixed waters containing a sodium-chloride com- ponent differ from the boiled waters in many respects. They are relatively high in total carbonate, most likely because the mixing process has prevented boiling and, therefore, degassing of the hot water. Further the mixed water is relatively high in calcium and magnesium and has low Na/K ratios which is explained by leaching sub- sequent to mixing (Arnórsson 1985). The mixed waters are devoid of H2S, most likely due to oxidation. In contrast to the mixed waters, waters in boiling hot springs have relatively high Na/K ratios. Na-K geother- rnometry temperatures for the boiling hot spring waters nre significantly below those of quartz. The cause is considered to be relatively effective removal of potassi- ntn from the boiling water in the upflow either by precip- itation of K-feldspar or adsorption on clay minerals. INTERPRETATION OF MAJOR GAS CHEMISTRY Samples of fumarole steam are available from the Landmannalaugar and Hrafntinnusker areas and a few samples from Reykjadalir (Fig. 1, Table 2). There is considerable variation in the gas compositions from the different areas and an almost complete overlap for the different gaseous constituents. The fumarole steam generally contains 0.2-0.4% to- tal gas by volume. Carbon dioxide always dominates (>70%) except for a few samples high in nitrogen and, therefore, taken to be atmospherically contaminated. Hydrogen sulphide accounts for 2-8% of the gas in most samples. Hydrogen concentrations are similar but more variable, most of the samples fall in the range 0-10%. Methane is below 0.6% except in one sample. Values below 0.2% are most common. Almost all the samples with methane in the range 0.2-0.6% are from the Hrafn- tinnusker area. Gas compositions comparable with those found in the Torfajökull field are known in some other high-temper- ature fields in Iceland (Arnórsson and Gunnlaugsson 1985). The concentrations of C02, H2S and H2 in the fumarole steam are considered to be governed by tem- perature dependent mineral-solution equilibria in a liq- uid dominated reservoir at depth and boiling, reactions and condensation in the upflow. The similarity of the gas composition at Torfajökull with those in some other fields in Iceland located in basaltic terrain suggests that the same secondary mineral assemblages control the gas TABLE 2. Analysis of representative samples of fumarole steam from Sample co2 H2S h2 no. 11 52.8 3.76 0.075 12 137.0 11.22 16.56 20 74.9 5.83 13.71 22 1133.5 52.78 198.2 29 177.9 11.38 20.29 31 62.6 5.95 1.86 35 170.0 6.87 4.61 36 1655.3 4.75 2.18 the Torfajökull field (mmoles/kg steam) ch4 o2 n2 Ar 0.010 0.00 5.92 0.222 0.342 0.02 1.20 0.082 0.163 0.14 1.80 0.062 0.954 0.36 6.22 0.209 0.747 1.46 20.33 0.821 0.209 2.22 16.88 0.553 0.346 0.02 2.18 0.136 0.179 0.15 3.05 0.137 Sample locations are shown in Fig. 1. The first 4 analyses arefrom Arnórsson and Gunnlaugsson (1985), nos. 098, 100,110 and 120 in their Table 6. 5
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